Has the US Government acknowledged this slight decline?
Yes, the federal government, specifically secretaries of education, have acknowledged the decline.
This article quotes secretary John B. King Jr. saying we are "losing ground" after the 2015 PISA, and quotes secretary Arne Duncan saying that the 2012 PISA was a "wake-up call against educational complacency and low expectations"
If yes, have any measures been taken to counterbalance this tendency?
Yes and no.
First and foremost is simply the extensive shared responsibility of education amongst the department of education and each individual state. For the US to have any unified measures taken in response to falling PISA scores would require either a large unified and coordinated effort by the states, or a more powerful department of education. This is not to say that the department of ed couldn't do anything, but that leads to my second point.
With changing administrations comes changing methods. Regardless of your opinion towards them/their strategies, Obama's and Trump's education outlook vary drastically. Obama pushed for common core which seeks to have a more unified federal approach by "commonizing" standards of excellence across the entire country as explained here. Alternatively, Trump has sought to decrease the power of the department of ed giving more power/responsibility to the individual states. (He's been quoted that he would eliminate the department if he could.)
I mention both presidents outlook toward education because although it may not be in direct response to PISA, it is their effort in creating a better educational experience in the USA, which theoretically should affect PISA scores.
So, yes, by virtue of trying new things(common core or decentralization) means the US government is responding to something they would like to be better. Also, no, it is not in direct response to the PISA scores